This proposal seeks to identify the event of neural development: birth, migration, differentiation, maintenance, or death, which estradiol influences during sexual differentiation of the zebra finch song system. Conspicuous, easily quantifiable, sexually dimorphic characteristics abound within the song system at both the cellular level and at the level of the system's gross organization. Treatment of genetically male zebra finches with exogenous 17 beta-estradiol in ovo prevents the development of the song system in the masculine direction. Using the techniques of 3H-thymidine autoradiography, Golgi-Cox impregnation and biochemical analysis of steroid binding macromolecules we intend to determine the timing of neuronal birth, migration, differentiation and death during song system development. Comparison of the timing of these ontogenetic events to the timing of the estradiol-sensitive period for sexual differentiation will be the first step in characterizing the biological basis of steroid-sensitive sexual differentiation.